1. Field of the Invention
The present invention relates to magnetic recording media, such as magnetic drums, magnetic tapes, magnetic disks and magnetic cards, and to a magnetic recording system. More specifically, the invention relates to a thin-film magnetic recording medium suited for ultra-high density magnetic recording, and to a magnetic recording system using such a magnetic recording medium.
2. Description of the Related Art
Remarkable advances in computers and information processing systems in recent years have made society increasingly information-oriented, and the amount of information handled by individuals is on the steady rise. As a result, there are growing demands for increased storage capacity and high-speed access of external storage devices of information processing systems. In particular, magnetic disk apparatuses--external storage devices suited for high-density recording--are under great demand for increased speed and capacity and reduced size.
Among the magnetic recording media used with a magnetic disk apparatus are a coated magnetic recording medium with oxide magnetic material powder applied to a substrate, and a thin-film magnetic recording medium with a thin film of metallic magnetic material deposited on a substrate by vacuum evaporation or sputtering. The thin-film magnetic recording medium has a high density of magnetic material in the film being recorded as compared with the coated magnetic recording medium, and is thus suited for recording at a higher density. For this reason, most of the magnetic disk apparatuses currently being manufactured employ thin-film magnetic recording media.
The substrate of the thin-film magnetic recording medium has commonly been a Ni-P plated Al-Mg alloy. As portable small computers have become more popular and impact resistance has become an important requirement, increasing numbers of harder reinforced glass substrates, crystallized glass substrates and carbon substrates are being used.
A widely known thin-film magnetic recording medium structure consists of an underlayer, a magnetic layer and a protective layer formed in that order on the substrate. Increasing the storage capacity of the magnetic disk apparatus requires increasing the coercivity of the thin-film magnetic recording medium.
In recent years, a bias sputtering method has attracted attention as a way of increasing coercivity. This method involves applying a negative DC bias voltage to a conductive substrate when depositing the underlayer and the magnetic layer and, upon completion of deposition of these layers, introducing argon (Ar) as a sputter gas to the surface of the substrate. This method is described in the IEEE Transaction on Magnetics, Vol. 26, 1282 (1990), and in Nippon Oyo Jiki Gakkaishi (Journal of the Japan Applied Magnetics Society), Vol. 16 (1992), page 541. For nonconductive substrates such as reinforced glass, on the other hand, an RF bias is normally used, as described in the Digest of Intermag. Conference, page EB-04, held on Apr. 13-16, 1993.
Nippon Oyo Jiki Gakkaishi, Vol. 17 (1993), supplement, No. S2, page 125, describes a sputtering method using krypton (Kr). This method, however, does not apply a bias voltage to the substrate, and fabricates the magnetic film by ordinary sputtering.
To realize a high recording density of more than 1 gigabits per square inch, the product of remanent magnetization and magnetic layer thickness of the magnetic recording medium should be kept below 150 G..mu.m to reduce the magnetizing field originating from the bit boundary. In this case, the coercivity must be at least 1600 Oe, and preferably at least 2000 Oe. It is, however, difficult to meet this requirement in a film deposited by ordinary sputtering.
As mentioned above, a bias sputtering method is known which applies a negative bias voltage to the substrate when forming the underlayer and the magnetic layer. A magnetic recording medium has yet to be produced by this method, however, which has read/write characteristics that satisfy the requirements of the above-mentioned high-density recording. Nor have thorough studies been conducted on the reliability characteristics of media manufactured by bias sputtering, such as corrosion resistance.
Further, in conventional thin-film media fabricating methods, for a hard nonconductive substrate that is suitable for use in one of the new small magnetic disk drives, only the RF bias sputtering method has been adopted. With this method, however, the manufacturing facility becomes large, and at the same time the production efficiency becomes significantly low. Additionally, the media thus fabricated do not have good enough read/write characteristics to enable high-density recording.
Moreover, when the product of remanent magnetization and magnetic layer thickness of the conventional magnetic recording medium is smaller than 150 G..mu.m, the sensitivity of a conventional inductive head, and that of a dual head having an inductive head for writing and a magnetoresistive (MR) head for reading (simply referred to as a dual head), may not be sufficiently high for acceptable operation. It is, therefore, desirable to use a magnetic head with higher reproduction sensitivity in conjunction with a signal processing circuit that is suitable for use with the magnetic head. Further, a signal modulation/demodulation circuit suitable for high density recording should be employed.